Ultrasonic motor

ABSTRACT

An ultrasonic motor  1  comprises a stator-side elastic member  30  having a rotationally symmetrical shape, a rotor-side elastic member  40  having a rotationally symmetrical shape, a stator-side piezoelectric element  31  for imparting vibrations to the elastic member  30,  a rotor-side piezoelectric element  41  for imparting vibrations to the elastic member  40,  an output shaft  10  rotating integrally with the elastic member  40,  a rotary transformer  60  for supplying power to the piezoelectric element  41  in non-contacting fashion, and housings  21  and  22  for accommodating therein the above-enumerated elements, wherein of three transformer circuits in the rotary transformer  60,  a middle transformer circuit C 2  and an outermost transformer circuit C 3  are used as a two-phase drive power transformer circuit. This construction achieves stable power supply to the rotor side.

TECHNICAL FIELD

The present invention relates to an ultrasonic motor wherein rotationaldisplacement waves are generated on press-contact faces of a stator androtor and a rotational motion is produced proportional to a frequencydifference between two rotational displacement waves. The invention alsorelates to an ultrasonic motor wherein a rotational motion is producedby generating a rotational displacement wave on a press-contact face ofa rotor.

BACKGROUND ART

A conventional traveling wave ultrasonic motor comprises an annularstator and an annular rotor which are pressed together in contactingfashion, and an annular piezoelectric element bonded to a back surfaceof the stator, wherein in operation, the piezoelectric element isexcited to generate a traveling wave on a press-contact face of thestator, and an elliptical motion at contact points is converted into arotational motion of the rotor (reference is made to Takashi Kenjo andToshiiku Sashida, “An Introduction to Ultrasonic Motors”, Sogo DenshiShuppan, 1991).

Ultrasonic motors of this type, compared with electromagnetic motors,have the advantage of being able to achieve compact and light weightconstruction owing to the lack of need for magnetic circuits such as acoil winding and an iron core and yet to produce high torque at slowrotational speed, and are commercially implemented in the fields ofcamera lens rotating mechanisms, robot actuators etc.

Since the conventional ultrasonic motor is of the type that the rotor isfrictionally driven by generating a traveling wave on the surface of thestator, in order to generate a traveling wave of large amplitude apiezoelectric element formed of a piezoelectric material having sharpresonance is driven to resonate at its resonant frequency. Consequently,when it is desired to control the rotational speed of the rotor in avariable manner, the traveling wave drive frequency or drive voltagemust be varied, but when the drive frequency or drive voltage of thepiezoelectric element is varied, the vibration output of thepiezoelectric element will drop abruptly or the torque will change. As aresult, variable control of the rotational speed is extremely difficultto achieve in the conventional ultrasonic motor and, in practice, theuse of this type of ultrasonic motor is limited to applications whereon-off control of a constant rpm output is performed, just like typicalDC motors.

In view of the above situation, a double drive-type ultrasonic motor hasbeen proposed in which piezoelectric elements are attached to both thestator and the rotor so that the rotational speed of the rotor can becontrolled in a continuously variable manner by the interaction betweenthe traveling wave on the stator and the traveling wave on the rotor(Japanese Unexamined Patent Publication JP-A 2-179281 (1990) JapaneseExamined Patent Publication JP-B2 2663164).

In this double drive-type ultrasonic motor, since the rotor is alsoprovided with a piezoelectric element, power must be supplied to therotating piezoelectric element by a suitable means.

In the ultrasonic motor described in JP-A 2-179281, the rotor and statorare housed within a case, an annular conductive plate is disposed on theupper surface of a rotor-side elastic member (the surface facing theinterior surface of the upper wall portion of the case), and aconductive brush which contacts the conductive plate in rubbing fashionis fixed to the interior surface of the case so as to face theconductive plate, wherein a signal of a given frequency is supplied tothe rotatable piezoelectric element at the rotor side via a power feedunit consisting of the conductive brush and the conductive plate.

In this ultrasonic motor, however, since the power feed unit is disposedbetween the rotor-side elastic member and the interior surface of theupper wall portion of the case, a particular space capable ofaccommodating the power feed unit has to be provided between therotor-side elastic member and the interior surface of the upper wallportion of the case, which leads to the problem that the size of themotor inevitably increases in the axial direction.

Furthermore, since the ultrasonic motor is configured so that theconductive brush contacts the conductive plate in rubbing fashion withthe conductive brush fixed to the interior surface of the cover and theconductive plate to the rotor-side elastic member, the cover and themotor mechanism must be matched (aligned) against each other, but thetask of matching is extremely difficult, resulting in the problem thatthe manufacturing increase.

On the other hand, for the traveling wave ultrasonic motor firstdescribed, a power feed method in which a rotary transformer or acombination of a slip ring and a brush is used is proposed as a methodapplicable to an ultrasonic motor where the piezoelectric element isattached to the rotor, not to the stator (Japanese Unexamined PatentPublication JP-A 4-71371 (1992)). However, the detailed configurationusing the rotary transformer is not presented, though the configurationusing the slip ring is described in detail therein.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an ultrasonic motorcapable of achieving stable power supply to a rotor side.

Another object of the invention is to provide an ultrasonic motor whichrealizes a compact motor construction, achieves reduction inmanufacturing costs, and is capable of regulating the rotation of therotor in a well controlled manner.

The present invention provides an ultrasonic motor comprising:

a stator-side elastic member having a rotationally symmetricalpress-contact face;

a rotor-side elastic member having a rotationally symmetricalpress-contact face facing the press-contact face of the stator-sideelastic member, the rotor-side elastic member being supported so as tobe angularly displaceable about a rotation symmetry axis;

a stator-side vibrating element for generating a rotational displacementwave of frequency Fs on the press-contact face of the stator-sideelastic member;

a rotor-side vibrating element for generating a rotational displacementwave of frequency Fr on the press-contact face of the rotor-side elasticmember, the rotor-side elastic member being angularly displaced at arotational speed proportional to a frequency difference ΔF between thefrequency Fs and the frequency Fr; and

a rotary transformer for supplying drive power having a phase differenceequivalent to N phases (N is an integer equal to or more than 2) to therotor-side vibrating element which is angularly displaced together withthe rotor-side elastic member,

the rotary transformer including more than N transformer circuitsconcentrically arranged, N transformer circuits which are sequentiallyarranged inwardly from an outermost one being used as drive powertransformer circuits.

According to the invention, by supplying power to the rotor-sidevibrating element using the rotary transformer, power feed with reducedpower loss and reduced mechanical loss can be achieved. Further, whendriving the rotor-side vibrating element with N-phase drive power, theunbalance between each phase of the drive power can be minimized bysupplying the drive power using the N transformer circuits starting fromthe outermost one out of the plurality of concentrically arrangedtransformer circuits. When the transformer circuits are arrangedconcentrically, the circumferential length of the core forming eachtransformer circuit varies according to the radius from the center ofrotation, that is, as the core diameter decreases, the radius ratiobetween adjacent cores increases, increasing the difference ininductance between adjacent cores and thus increasing the difference intransformer transfer efficiency. On the other hand, the outer thetransformer circuits are located, the smaller the unbalance betweenadjacent cores, and therefore, such transformer circuits are suitablefor supplying N-phase drive power.

It is preferable that the rotary transformer is disposed on the rotationsymmetry axis side with respect to the press-contact faces of thestator- and rotor-side elastic members, and in that case, since asufficient space for accommodating the rotary transformer can besecured, not only the overall size of the motor can be reduced, but alsoa relatively large-sized rotary transformer can be used, thus makinghigh-output, high-efficiency power feed possible. Especially, when eachelastic member is formed in a ring shape, the space saving effect isfurther enhanced since the rotary transformer can be accommodated insidethe ring.

The invention is also characterized in that an innermost one of thetransformer circuits is used as a detection signal transformer circuitfor transmitting a detection signal resulting from the detection of therotational displacement wave generated on the press-contact face of therotor-side elastic member.

According to the invention, since the detection signal resulting fromthe detection of the rotational displacement wave on the rotor-sideelastic member is a single-phase signal, one transformer circuit shouldsuffice for the signal transmission. Accordingly, even if, of theplurality of concentrically arranged transformer circuits, the innermosttransformer circuit, which has large characteristic differences betweenadjacent cores is used for that purpose, the “unbalance” problem doesnot occur. This also allows the outer and more suitable transformercircuits to be used for other circuit systems where the balance isimportant.

The invention is also characterized in that the stator-side vibratingelement and the rotor-side vibrating element, respectively, arerotationally symmetrical piezoelectric elements attached to surfaces ofthe stator-side and rotor-side elastic members, which surfaces areopposite to the press-contact faces thereof,

first and second drive electrodes for two-phase driving and a monitorelectrode for detecting a vibrating wave are formed on a surface of eachof the piezoelectric elements, and

the first and second drive electrodes are respectively connected to thedrive power transformer circuits, and the monitor electrode is connectedto the detection signal transformer circuit.

According to the invention, by using rotationally symmetricalpiezoelectric elements as the stator- and rotor-side vibrating elements,a rotational displacement wave can be generated efficiently on thepress-contact face of each of the stator- and rotor-side elasticmembers. The rotational displacement wave here is a surface wave createdby a given point on the press-contact face elliptically moving within aplane containing the propagation direction of the wave and the directionof a plane normal. When there occurs a frequency difference betweenopposing points, a velocity difference occurs between the stator-sideelastic member and the rotor-side elastic member, resulting in thegeneration of a torque for rotating the rotor-side elastic member.

Further, by forming the first and second drive electrodes for two-phasedriving on the surface of each piezoelectric element, a rotationaldisplacement wave can be generated along the circumference direction ofthe piezoelectric element, and by detecting the vibrating wave with themonitor electrode, feedback can be provided to the drive circuit for thepiezoelectric element. Thus, the rotary transformer includes two drivepower transformer circuits and one detection signal transformer circuitto provide independent power supplies and achieve signal transmission.

The invention is also characterized in that a non-magnetic material forsuppressing magnetic coupling is interposed between a detection coreforming the detection signal transformer circuit and a drive coreforming the drive power transformer circuits.

According to the invention, by providing the non-magnetic material tosuppress the magnetic coupling between the detection core and the drivecore, crosstalk from the drive signal lines to the detection signal lineof the piezoelectric element can be greatly reduced. Since the detectionsignal from the monitor electrode is used as a feedback signal to thedrive circuit for the piezoelectric element, introduction of noise wouldcause an unstable condition in the drive circuit. Furthermore, sincethis detection signal is obtained using the piezoelectric effect, theresulting high impedance output is susceptible to noise. Therefore, bysuppressing the magnetic coupling with the drive core which transmitslarge power, the S/N ratio of the detection signal transmitting in thedetection core can be improved.

The present invention also provides an ultrasonic motor comprising:

a stator-side elastic member having a rotationally symmetricalpress-contact face;

a rotor-side elastic member having a rotationally symmetricalpress-contact face facing the press-contact face of the stator-sideelastic member, the rotor-side elastic member being supported so as tobe angularly displaceable about a rotation symmetry axis;

a rotor-side vibrating element for generating a rotational displacementwave on the press-contact face of the rotor-side elastic member, therotor-side elastic member being angularly displaced by the rotationaldisplacement wave; and

a rotary transformer for supplying drive power having a phase differenceequivalent to N phases (N is an integer equal to or more than 2) to therotor-side vibrating element which is angularly displaced together withthe rotor-side elastic member,

the rotary transformer including more than N transformer circuitsconcentrically arranged, N transformer circuits of the more than Ntransformer circuits which are sequentially arranged inwardly from anoutermost one being used as drive power transformer circuits.

According to the invention, by supplying power to the rotor-sidevibrating element using the rotary transformer, power feed with reducedpower loss and reduced mechanical loss can be achieved. Further, whendriving the rotor-side vibrating element with N-phase drive power, theunbalance between each phase of the drive power can be minimized bysupplying the drive power using the N transformer circuits starting fromthe outermost one out of the plurality of concentrically arrangedtransformer circuits. When the transformer circuits are arrangedconcentrically, the circumferential length of the core forming eachtransformer circuit varies according to the radius from the center ofrotation, that is, as the core diameter decreases, the radius ratiobetween adjacent cores increases, increasing the difference ininductance between adjacent cores and thus increasing the difference intransformer transfer efficiency. On the other hand, the outer thetransformer circuits are located, the smaller the unbalance betweenadjacent cores, and therefore, such transformer circuits are suitablefor supplying N-phase drive power.

The present invention further provides an ultrasonic motor comprising:

a stator-side elastic member having a rotationally symmetricalpress-contact face;

a rotor-side elastic member having a rotationally symmetricalpress-contact face facing the press-contact face of the stator-sideelastic member, the rotor-side elastic member being supported so as tobe angularly displaceable about a rotation symmetry axis;

a stator-side vibrating element for generating a rotational displacementwave of frequency Fs on the press-contact face of the stator-sideelastic member;

a rotor-side vibrating element for generating a rotational displacementwave of frequency Fr on the press-contact face of the rotor-side elasticmember, the rotor-side elastic member being angularly displaced at arotational speed proportional to a frequency difference ΔE between thefrequency Fs and the frequency Fr; and

a rotary transformer, disposed on the rotation symmetry axis side withrespect to the press-contact faces of the stator- and rotor-side elasticmembers, for supplying power to the rotor-side vibrating element whichis angularly displaced together with the rotor-side elastic member,

a step-up ratio Nr of the rotary transformer being larger than 1.

According to the invention, by supplying power to the rotor-sidevibrating element using the rotary transformer, power feed with reducedpower loss and reduced mechanical loss can be achieved. Further, bymounting the rotary transformer on the rotation symmetry axis side withrespect to the press-contact faces of the stator- and rotor-side elasticmembers, the overall size of the motor can be reduced since a sufficientspace for accommodating the rotary transformer can be secured. Sincethis also allows the use of a relatively large-sized rotary transformer,high-output, high-efficiency power feed can be achieved. Especially,when each elastic member is formed in a ring shape, the space savingeffect is further enhanced since the rotary transformer can beaccommodated inside the ring.

When piezoelectric elements are used as the stator- and rotor-sidevibrating elements, relatively high drive voltages must be supplied;therefore, when a rotary transformer having a step-up ratio Nr largerthan 1 is used, the rotary transformer can be made to also function as astep-up transformer. As a result, while it was common to use a step-upfixed transformer in conventional traveling wave ultrasonic motors, inthe present invention the rotary transformer can be used as a substitutefor the fixed transformer for the rotor-side vibrating element; thisserves to reduce the number of components.

The invention is also characterized in that the ultrasonic motor furthercomprises a fixed transformer for supplying power to the stator-sidevibrating element and a ratio Nr/Ns between the step-up ratio Nr of therotary transformer and the step-up ratio Ns of the fixed transformersatisfies a relation of 0.5≦Nr/Ns ≦2.

According to the invention, by setting the ratio Nr/Ns between thestep-up ratio Nr of the rotary transformer and the step-up ratio Ns ofthe fixed transformer within the range of 0.5 to 2, the unbalance of thesupply power or drive voltages to the stator- and rotor-side vibratingelements can be eliminated.

When identical drive circuits are used for both the stator-sidevibrating element and the rotor-side vibrating element, it is preferablethat the step-up ratios Nr and Ns of the respective circuits are madesubstantially the same, and in that case, the operation of thestator-side vibrating element can be made to substantially match that ofthe rotor-side vibrating element.

The present invention also provides an ultrasonic motor comprising:

a stator-side elastic member having a rotationally symmetricalpress-contact face;

a rotor-side elastic member having a rotationally symmetricalpress-contact face facing the press-contact face of the stator-sideelastic member, the rotor-side elastic member being supported so as tobe angularly displaceable about a rotation symmetry axis;

a rotor-side vibrating element for generating a rotational displacementwave on the press-contact face of the rotor-side elastic member, therotor-side elastic member being angularly displaced by the rotationaldisplacement wave; and

a rotary transformer for supplying power to the rotor-side vibratingelement which is angularly displaced together with the rotor-sideelastic member,

a step-up ratio Nr of the rotary transformer being larger than 1.

According to the invention, by supplying power to the rotor-sidevibrating element using the rotary transformer, power feed with reducedpower loss and reduced mechanical loss can be achieved. Further, it ispreferable that the rotary transformer is disposed on the rotationsymmetry axis side with respect to the press-contact faces of thestator- and rotor-side elastic members; by so arranging, the overallsize of the motor can be reduced since a sufficient space foraccommodating the rotary transformer can be secured. Since this alsoallows the use of a relatively large-sized rotary transformer,high-output, high-efficiency power feed can be achieved. Especially,when each elastic member is formed in a ring shape, the space savingeffect is further enhanced since the rotary transformer can beaccommodated inside the ring.

When a piezoelectric element is used as the stator-side vibratingelement, a relatively high drive voltage must be supplied; therefore,when a rotary transformer having a step-up ratio Nr larger than 1 isused, the rotary transformer can be made to also function as a step-uptransformer. As a result, while it was common to use a step-up fixedtransformer in conventional traveling wave ultrasonic motors, in thepresent invention the rotary transformer can be used as a substitute forthe fixed transformer for the rotor-side vibrating element; this servesto reduce the number of components.

The invention further provides an ultrasonic motor comprising a statorhaving a stator-side elastic member and a stator-side vibrator attachedto the stator-side elastic member; and a rotor having a rotor-sideelastic member facing and pressed against the stator-side elastic memberand a rotor-side vibrator attached to the rotor-side elastic member, thevibrators being caused to vibrate by supplying signals of prescribedfrequency to the respective vibrators, the rotor being driven by meansof traveling waves generated on the press-contact faces of therespective elastic members by the vibrations, wherein the stator-sideelastic member and the rotor-side elastic members are respectivelyprovided with recessed portions opposing each other and are formed intosubstantially the same shape, and a power feed unit for supplying thesignal of the prescribed frequency to the rotor-side vibrator isdisposed within a space formed between the opposing recessed portions.

According to the invention, since the two elastic members are madesubstantially identical in shape, the stator and rotor can beconstructed to have the same natural frequency of vibration.Accordingly, when signals whose frequencies are equal to or nearly equalto the resonant frequency of the stator and rotor are supplied to therespective vibrators, the traveling waves generated on the press contactfaces of the elastic members lock (engage) with each other, just asgears engage with each other, and the rotor is held in the stoppedcondition; when the phase of one signal is shifted in the positive ornegative direction with respect to the phase of the other signal, theengagement lock position shifts in the forward or backward direction,and by repeating this shift operation, the rotor is caused to rotate inthe forward or backward direction. That is, when the two elastic membersare formed in substantially the same shape, the rotation of the rotorcan be regulated in a well controlled manner.

Further, since the space formed between the opposing recessed portionsof the elastic members is utilized to accommodate the power feed unitfor supplying the signal of the prescribed frequency to the rotor-sidevibrator, there is no need to particularly provide a space foraccommodating the power feed unit along the axial direction of themotor, and the size of the motor can thus be reduced in the axialdirection compared with the prior art. Furthermore, since this alsoeliminates the need for the matching between the cover and the motormechanism, which is difficult to achieve, the manufacturing costs can bereduced. Moreover, since the same components can be used between therespective elastic members, their costs can be reduced.

A variety of configurations are possible for the power feed unit, forexample, a rotary transformer may be employed.

It is also possible to employ as the power feed unit a brush contactpower feed unit comprising a conductive brush attached to either thestator or rotor side and a conductors provided at the other side, forcontacting the conductive brushes in rubbing fashion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross sectional view showing the construction of oneembodiment according to the present invention, and

FIG. 1B is an enlarged cross sectional view of a rotary transformer 60in FIG. 1A;

FIGS. 2A and 2B are exploded perspective views illustrating an assemblyprocedure;

FIGS. 3A and 3B are exploded perspective views illustrating an assemblyprocedure;

FIG. 4 is an enlarged cross sectional view of the rotary transformer 60shown in FIGS. 2 and 3;

FIG. 5A is a plan view showing the polarization state of a piezoelectricelement 31, and

FIG. 5B is a plan view showing the electrode configuration of thepiezoelectric element 31;

FIGS. 6A-6G are graphs showing how a traveling wave is generated: FIG.6A is a diagram showing the piezoelectric element 31 expanded in astraight line form, FIG. 6G is a diagram showing voltage waveformsapplied to drive electrodes 31 a and 31 b, and FIGS. 6B to 6F arediagrams showing the waveforms of the traveling wave at times t0 to t4in FIG. 6G, respectively;

FIG. 7 is a block diagram showing one example of a drive control circuitfor an ultrasonic motor;

FIG. 8 is a block diagram showing another example of a drive controlcircuit for an ultrasonic motor;

FIG. 9 is a graph showing the inductance characteristics of the rotarytransformer 60;

FIG. 10 is a cross sectional side elevation view showing an ultrasonicmotor according to a second embodiment;

FIG. 11 is a plan view showing one side of a stator-side vibrator or arotor-side vibrator;

FIG. 12 is a plan view showing the other side of the stator-sidevibrator or the rotor-side vibrator;

FIG. 13 is a cross sectional view of a stator/rotor contact portion cutalong the circumference direction thereof when traveling waves aregenerated;

FIG. 14 is a cross sectional side elevation view showing an ultrasonicmotor according to a third embodiment; and

FIG. 15 is a cross sectional side elevation view showing an ultrasonicmotor according to a fourth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of an ultrasonic motor according to the presentinvention will be described below with reference to the accompanyingdrawings. Throughout the drawings, like elements or elements identicalin function are designated by like reference numerals, and thedescription of such elements, once given, will not be repeatedthereafter.

FIG. 1A is a cross sectional view showing the construction of oneembodiment according to the present invention, and FIG. 1B is anenlarged cross sectional view of a rotary transformer 60 in FIG. 1A.FIGS. 2 and 3 are exploded perspective views illustrating assemblyprocedures, and FIG. 4 is an enlarged cross sectional view of the rotarytransformer 60 in FIGS. 2 and 3.

An ultrasonic motor 1 comprises a stator-side elastic member 30 having arotationally symmetrical shape, a rotor-side elastic member 40 having arotationally symmetrical shape, a stator-side piezoelectric member 31for imparting vibrations to the elastic member 30, a rotor-sidepiezoelectric member 41 for imparting vibrations to the elastic member40, an output shaft 10 rotating integrally with the elastic member 40,the rotary transformer 60 for supplying power to the piezoelectricelement 41 in non-contacting fashion, and housings 21 and 22 foraccommodating therein the above-enumerated elements.

The housing 21 is shaped into a cylindrical box consisting of a bottomand an encircling side wall, in the center of which bottom a throughhole is formed, and a hollow bearing holder 20 having a T-shaped crosssection is fitted into the through hole. Steps are formed in the upperand lower sections of the interior surface of the bearing holder 20, andbearings 15 and 16 for rotatably supporting the output shaft 10 arefixedly disposed on the respective steps, one being spaced apart fromthe other by a predetermined distance.

The output shaft 10 is inserted so that the inner rings of the bearings15 and 16 substantially fit around the output shaft 10. The bearing 16is held in position by abutting against a spacer 13 disposed on the stepface of a thick shaft portion 10 b in the center of the output shaft 10,while the bearing 15 is held in position by a spacer 12 fitted onto theoutput shaft 10 from one end thereof, around which output shaft acircumferential groove 10 a is formed in which a stopper 11 such as anE-ring is fitted to support the output shaft 10 integrally with theinner rings of the bearings 15 and 16.

The elastic member 30 is disposed on the upper surface of the bearingholder 20, and is centered by being held in substantial engagement withthe outer periphery of the bearing 16.

The elastic member 30 comprises a thick wall portion having on an uppersurface thereof an annular press-contact face 30 a, a thin wall portion30 b supporting the thick wall portion from inside, and a mountingportion substantially fitted on an inner spacer 14, and is configured soas to allow the thick wall portion to vibrate in its thicknessdirection. The piezoelectric element 31 which has an annular disc shapeis bonded to the lower surface of the thick wall portion opposite thepress-contact face 30 a. The press-contact face 30 a has a protrudingcross section so that the vibrations produced in the piezoelectricelement 31 can be efficiently concentrated at the protrusion, and aliner 32 formed of a low friction material such as a fluororesin isfixed to the upper surface of the protrusion.

Similarly to the elastic member 30, the rotor-side elastic member 40comprises a thick wall portion having on a lower surface thereof anannular press-contact face 40 a, a thin wall portion 40 b supporting thethick wall portion from inside, and a mounting portion for transmittinga torque to the output shaft 10, and is configured so as to allow thethick wall portion to vibrate in its thickness direction. Thepiezoelectric element 41 which has an annular disc shape is rigidlybonded to the upper surface of the thick wall portion opposite thepress-contact face 40 a. The press-contact face 40 a has a protrudingcross section so that the vibrations produced in the piezoelectricelement 41 can be efficiently concentrated at the protrusion, and aliner 42 formed of a low friction material such as a fluororesin isfixed to the lower surface of the protrusion.

Preferably, the elastic members 30 and 40 are formed of a materialhaving a low damping force, for example, a metal such as iron or brass.

An annular disc shaped spring receiving plate 45 is disposed on themounting portion of the elastic member 40. Centering is achieved withthe inside of the spring receiving plate 45 substantially engaging witha stepped portion 10 d of the output shaft 10. An engaging plate 43having two protrusions 43 a is disposed on the inner portion of theupper surface of the spring receiving plate 45. With the protrusions 43a engaging with notches 10 g cut in a base end portion 10 e of theoutput shaft 10, the torque rotating the elastic member 40 istransmitted to the output shaft 10.

A plate spring 44 is attached to the base end face of the output shaft10, and centering is achieved with the through hole formed in the centerof the plate spring 44 substantially engaging on the protrusion 10 fformed at the center of the base end of the output shaft 10.

The plate spring 44 comprises a center disc and a plurality of radiallyextending swinging ends, and the force for pressing the elastic members30 and 40 together is generated with the swinging ends applying uniformpressure to the spring receiving plate 45.

The rotary transformer 60 is disposed within a ring-shaped spaceprovided inwardly of the thick wall portions of the annular elasticmembers 30 and 40 and between the thin wall portion/mounting portionsections of the elastic members 30 and 40.

The rotary transformer 60 comprises an annular stator-side core 61 andan annular rotor-side core 62 disposed opposite each other with aprescribed gap provided therebetween. The stator-side core 61 isdisposed on the annular spacer 14. The rotor-side core 62 is disposed onthe step face of the stepped portion 10 d and substantially fittedaround a thick shaft portion 10 c whose diameter is slightly larger thanthat of the thick shaft portion 10 b of the output shaft 10, so that thecore 62 rotates integrally with the output shaft 10.

Next, an electrical system will be described. FIG. 5A is a plan viewshowing the polarization state and electrode configuration of thepiezoelectric element 31, and FIG. 5B is a plan view showing theelectrode configuration on the side of the piezoelectric element 31opposite from the side thereof shown in FIG. 5A. The piezoelectricelement 41 has the same polarization state and electrode configurationas the piezoelectric element 31.

When a driving electric field is applied in the thickness direction, theannular piezoelectric element 31 expands and contracts in the thicknessand circumference directions, and produces vibrations with theexpand/contract motions rapidly alternating, and a rotationaldisplacement wave such as a traveling wave or standing wave is createdby changing the phase of the vibration along the circumferencedirection. In the figures is shown the case where a 5-perioddisplacement wave is generated around the circumference, and as shown inFIG. 5A, there are formed two groups A and B, each consisting of fourλ/2 polarization regions alternately polarized in opposite directionsfor every λ/2, and a λ/4 polarization region and a 3λ/4 polarizationregion are respectively interposed between the groups A and B, where λis the wavelength of the displacement wave. For the thus arrangedpolarization regions and electrodes, a drive electrode 31 a, a driveelectrode 31 b, and a monitor electrode 31 c are formed corresponding tothe group A, the group B, and the λ/4 polarization region, respectively.Here, the elastic member 30 on which the piezoelectric element 31 isdisposed functions as an annular common electrode facing the electrodesA, B, λ/4, and 3λ/4, and the elastic member 30 is grounded via thehousing 21. Likewise, the elastic member 40 functions as a commonelectrode for the piezoelectric element 41, and is grounded via thehousing 21.

When an electric field is applied across the thickness of thepiezoelectric element 31 (for example, perpendicular to the plane of thefigure and into the plane), the piezoelectric element 31 expands andcontracts in the thickness and circumference directions due to theelectrostrictive effect, in the positive polarization regions markedwith “+”, the thickness increases, while in the negative polarizationregions marked with “−”, the thickness decreases. For example, when apositive voltage is applied to the drive electrode 31 a corresponding tothe group A, the displacement direction changes every λ/2, and a wavefor two wavelengths is created with nodes of the wave formed atboundaries between the respective polarization regions. When the appliedvoltage to the drive electrode 31 a is varied at a frequency in theultrasonic frequency range, the wave generated from the group Apropagates through the entire piezoelectric element 31. Likewise, whenan AC voltage in the ultrasonic frequency range is applied to the driveelectrode 31 b corresponding to the group B, a wave for two wavelengthspropagates through the entire piezoelectric element 31, and thus thewave from the group A and the wave from the group B are superimposed oneon top of the other. Accordingly, by applying a cosine wave to the driveelectrode 31 a and a sine wave to the drive electrode 31 b with theirphases shifted 90 degrees relative to each other, a traveling wavepropagating in a given direction can be generated.

FIG. 6 is a graph showing how the traveling wave is generated. FIG. 6Ais a diagram showing the piezoelectric element 31 expanded in a straightline form, and FIG. 6G shows voltage waveforms applied to the driveelectrodes 31 a and 31 b. FIGS. 6B to 6F show the waveforms of thetraveling wave at times t0 to t4 in FIG. 6G. At time t0, only the groupA is generating a surface wave of amplitude 1, which propagates throughthe entire ring. At time t1, the groups A and B are generating surfacewaves of amplitude 1/{square root over (2)}, 90 degrees apart in phase,and the sum wave is shifted to the left by λ/8 compared with thewaveform at time t0. At time t2, only the group B is generating asurface wave of amplitude 1, which is shifted to the left by λ/8compared with the wave format time t1. Similarly, at times t3 and t4,the wave is shifted to the left by λ/8 each time, and it is thus shownthat the traveling wave moving to the left with time is generated.

Conversely, when a sine wave is applied to the drive electrode 31 a anda cosine wave to the drive electrode 31 b with their phases shifted 90degrees in the opposite direction, a traveling wave moving in thedirection opposite to that described above can be generated. In thisway, by driving the respective electrodes by relating the spatial phasedifference between the groups A and B to the temporal phase differencebetween the respective drive waveforms, a traveling wave moving in thedesired direction can be generated. Since the piezoelectric element 31is rigidly bonded to the elastic member 30, the expand/contract motionsof the piezoelectric element 31 are translated into a wave having aprescribed amplitude. Then, the two waves differing in phase are summedtogether to create a traveling wave, and this traveling wave istransmitted through the elastic member 30, generating a traveling waveon the press-contact face 30 a. Similarly, the traveling wave created bythe expand/contract motions of the piezoelectric element 41 istransmitted through the elastic member 40, generating a traveling waveon the press-contact face 40 a.

The vibrating waveform of the traveling wave generated in thepiezoelectric element 31 due to the piezoelectric effect is detected asan electrical signal by the monitor electrode 31 c. The drive electrodes31 a and 31 b and the monitor electrode 31 c are connected to lines PA,PB, and PC, respectively, which are leaded out through a lead-out holeformed in the housing 21.

In the case of the piezoelectric element 41 also, the drive electrodes41 a and 41 b and the monitor electrode 41 c are connected to lines QA,QB, and QC, respectively, in this case via the rotary transformer 60.

FIG. 1 shows an example in which the rotary transformer 60 is configuredas a two-phase drive transformer and includes three transformer circuitscorresponding to the lines QA, QB, and QC. As shown in FIG. 1B, fiveannular protrusions are formed in concentric fashion on each of thestator-side core 61 and the rotor-side core 62, of which protrusions,outer three protrusions function as drive-side cores 61 a and 62 aforming the transformer circuits for the lines QA and QB, and inner twoprotrusions function as detection-side cores 61 c and 62 c forming thetransformer circuit for the line QC. Each annular protrusion is providedwith grooves spaced apart from one another by a prescribed angle, thusforming a plurality of sub protrusions, and a coil is disposed so as tobe wound around each sub protrusion, and in this arrangement, power feedand signal transmission are accomplished in non-contacting fashion bythe magnetic coupling between the opposing annular protrusions.

Further, the drive-side core 61 a, 62 a and the detection-side core 61c, 62 c may be separated by inserting therebetween a separator 61 b, 62b formed of a non-magnetic material such as a plastic material in orderto suppress magnetic coupling between the respective cores. Suchseparators 61 b, 62 b may be inserted not only between the drive-sidecore 61 a, 62 a and the detection-side core 61 c, 62 c, but also betweenthe respective lines QA, QB, and QC. By providing such non-magneticseparators 61 b, 62 b, magnetic coupling between the respectivetransformer circuits can be suppressed. The separators 61 b, 62 b may beformed by filling adhesive material.

On the other hand, FIGS. 2 to 4 show an example in which the rotarytransformer 60 is configured as a three-phase drive transformer andincludes four transformer circuits corresponding to lines SA, SB, SC,and SD. As shown in FIG. 4, six annular protrusions are formed inconcentric fashion on each of the stator-side core 61 and the rotor-sidecore 62, of which protrusions, outer four protrusions function as thedrive-side cores 61 a and 62 a forming the transformer circuits for thelines SA, SB, and SC, and inner two protrusions function as thedetection-side cores 61 c and 62 c forming the transformer circuit forthe line SD. Each annular protrusion is provided with grooves spacedpart from one another by a prescribed angle, thus forming a plurality ofsub protrusions, and a coil is disposed so as to be wound around eachsub protrusion, and in this arrangement, power feed and signaltransmission are accomplished in non-contacting fashion by the magneticcoupling between the opposing annular protrusions.

Further, the drive-side core 61 a, 62 a and the detection-side core 61c, 62 c may be separated by inserting therebetween a separator 61 b, 62b formed of a non-magnetic material such as a plastic material in orderto suppress magnetic coupling between the respective cores.

FIG. 7 is a block diagram showing one example of a drive control circuitfor the ultrasonic motor. In this circuit example, the stator isconstructed as the main drive part and the rotor as the driven part. Theelastic members 30 and 40 in the ultrasonic motor 1 are pressed togetherinto contact with each other and, in this condition, are electricallygrounded via the output shaft 10, the housing 21, etc. The driveelectrodes 31 a, 31 b and monitor electrode 31 c of the piezoelectricelement 31 attached to the stator-side elastic member 30 are connectedto the lines PA, PB, and PC, respectively. Step-up transformers 70 and71 for generating high voltages necessary for piezoelectric driving areinserted between the lines PA, PB and the corresponding electrodes.Here, the step-up transformers 70 and 71 are fixed to a circuit board orthe like.

The drive electrodes 41 a, 41 b and monitor electrode 41 c of thepiezoelectric element 41 attached to the rotor-side elastic member 40are connected to the lines QA, QB, and QC, respectively, with the rotarytransformer 60 interposed therebetween. When the step-up ratio Nr of therotary transformer 60 is set to 1 or lower, a step-up transformer forgenerating a high voltage for piezoelectric driving has to be providedseparately, but by setting the step-up ratio Nr of the rotarytransformer 60 larger than 1, the step-up transformer at the rotor sidecan be omitted because the rotary transformer 60 can serve the samefunction as the step-up transformers 70 and 71.

Further, it is preferable that the electrical and mechanicalcharacteristics are made substantially the same between the stator sideand the rotor side so that the elastic members 30 and 40 will generatetraveling waves of equal amplitudes, and the unbalance can be eliminatedwhen the ratio Nr/Ns between the step-up ratio Nr of the rotarytransformer 60 and the step-up ratio Ns of the step-up transformers 70,71 is within the range of 0.5≦Nr/Ns≦2, and more preferably Ns≈Nr.

A frequency control oscillator 82 at the stator side outputs anultrasonic drive signal of frequency Fs (for example, a sine wave or apulse wave) which, after being amplified by an amplifier 86, is outputon the line PA and converted by the step-up transformer 70 into a highvoltage, which is applied to the drive electrode 31 b of thepiezoelectric element 31. The ultrasonic drive signal from the frequencycontrol oscillator 82 is also supplied to an amplifier 88 via a phaseshifter 84 which shifts the phase by 90 degrees, and the signalamplified by the amplifier 88 is output on the line PB and converted bythe step-up transformer 71 into a high voltage, which is applied to thedrive electrode 31 a of the piezoelectric element 31.

A detection signal generated at the monitor electrode 31 c of thepiezoelectric element 31 is input to a wave shaping limiter circuit 90via the line PC, and returned as a feedback signal to the frequencycontrol oscillator 82. The frequency control oscillator 82 isconstructed, for example, from a PLL (Phase Locked Loop) circuitcomprising a VCO (voltage controlled oscillator), a phase comparator,and an LPF (low pass filter), and operates in self-driving fashion.

At the rotor side, based on the ultrasonic drive signal of frequency Fsoutput from the frequency control oscillator 82, a frequency controlcircuit 83 outputs an ultrasonic drive signal of frequency Fr (forexample, a sine wave or a pulse wave) which, after being amplified by anamplifier 87, is output on the line QA and converted by the rotarytransformer 60 into a high voltage, which is applied to the driveelectrode 41 a of the piezoelectric element 41. The ultrasonic drivesignal from the frequency control circuit 83 is also supplied to anamplifier 89 via a phase shifter 85 which shifts the phase by 90degrees, and the signal amplified by the amplifier 89 is output on theline QB and converted by the rotary transformer 60 into a high voltage,which is applied to the drive electrode 41 b of the piezoelectricelement 41.

A detection signal generated at the monitor electrode 41 c of thepiezoelectric element 41 is not used when the rotor side is the drivenside. To detect an angular error between the rotor and stator, thefeedback signals at the rotor and stator sides are used to detect thephase difference between the signals.

When a command is input from an external host device 80 such as acomputer, the frequency control circuit 83 interprets the command andcontrols the frequency Fr based on the frequency Fs from the frequencycontrol oscillator 82.

For example, when the command from the external host device 80 is arotational speed command, the frequency Fr is controlled by adding afrequency difference ΔF corresponding to the commanded rotational speedto the signal from the oscillator 82 so that the frequency difference ΔFis maintained. Then, the piezoelectric elements 31 and 41 generatevibrations of frequencies Fs and Fr, respectively, as described above,and these vibrations propagate through the respective elastic members 30and 40, generating on the respective press-contact faces 30 a and 40 arotational displacement waves WA and WB having displacement componentsalong the circumference direction. Since the rotational displacementwaves WA and WB on the press-contact faces 30 a and 40 a are shifted infrequency by an amount equal to the frequency difference ΔF, therotor-side elastic member 40 rotates relative to the stator-side elasticmember 30, and the thus generated rotational torque is taken from theoutput shaft 10 shown in FIG. 1. Since the rotational speed of theelastic member 40 varies in proportion to the frequency difference ΔF,the rotational speed of the ultrasonic motor 1 can be controlled withhigh precision by precisely controlling the frequency difference ΔF bymeans of the frequency control circuit 83.

Furthermore, when the frequency difference ΔF relative to the frequencyFs is reduced to zero by controlling the frequency Fr, the ultrasonicmotor 1 comes to rest, with the pressing force between the elasticmembers 30 and 40 acting as the motor holding torque, thus eliminatingthe need for a brake mechanism.

When the command from the external host device 80 is a rotation anglecommand, the prescribed rotational speed corresponding to the frequencydifference ΔF is maintained for a time corresponding to the rotationangle command, and upon the expiration of the time, the motor is causedto stop by controlling the frequency Fr so as to match the frequency Fs.Accordingly, rotation angle control like a stepping motor can also beaccomplished.

In this way, by receiving the signal from the oscillator 82 and bycontrolling the frequency Fr appropriately, the frequency controlcircuit 83 can control the rotational speed, rotation angle, rotationaldirection, etc. of the ultrasonic motor 1 as desired.

FIG. 8 is a block diagram showing another example of a drive controlcircuit for the ultrasonic motor. In this circuit example, the rotor isconstructed as the main drive part and the stator as the driven part.

The frequency control oscillator 82 outputs an ultrasonic drive signalof frequency Fr (for example, a sine wave or a pulse wave) which, afterbeing amplified by the amplifier 87, is output on the line QA andconverted by the rotary transformer 60 into a high voltage, which isapplied to the drive electrode 41 a of the piezoelectric element 41. Theultrasonic drive signal from the frequency control oscillator 82 is alsosupplied to the amplifier 89 via the phase shifter 85 which shifts thephase by 90 degrees, and the signal amplified by the amplifier 89 isoutput on the line QB and converted by the rotary transformer 60 into ahigh voltage, which is applied to the drive electrode 41 b of thepiezoelectric element 41.

The detection signal generated at the monitor electrode 41 c of thepiezoelectric element 41 is input to the wave shaping limiter circuit 90via the line QC, and returned as a feedback signal to the frequencycontrol oscillator 82. The frequency control oscillator 82 isconstructed, for example, from a PLL (Phase Locked Loop) circuitcomprising a VCO (voltage controlled oscillator), a phase comparator,and an LPF (low pass filter), and operates in self-driving fashion.

At the stator side, based on the ultrasonic drive signal of frequency Froutput from the frequency control oscillator 82, the frequency controlcircuit 83 outputs an ultrasonic drive signal of frequency Fs (forexample, a sine wave or a pulse wave) which, after being amplified bythe amplifier 86, is output on the line PA and converted by the step-uptransformer 70 into a high voltage, which is applied to the driveelectrode 31 b of the piezoelectric element 31. The ultrasonic drivesignal from the frequency control circuit 83 is also supplied to theamplifier 88 via the phase shifter 84 which shifts the phase by 90degrees, and the signal amplified by the amplifier 88 is output on theline PB and converted by the step-up transformer 71 into a high voltage,which is applied to the drive electrode 31 a of the piezoelectricelement 31.

The detection signal generated at the monitor electrode 31 c of thepiezoelectric element 31 is not used when the stator is the driven side.To detect an angular error between the rotor and stator, the feedbacksignals at the rotor and stator sides are used to detect the phasedifference between the signals.

When a command is input from the external host device 80 such as acomputer, the frequency control circuit 83 interprets the command andcontrols the frequency Fs based on the frequency Fr from the frequencycontrol oscillator 82. In this way, the rotational speed, rotationangle, and rotational direction, etc. of the ultrasonic motor 1 can becontrolled as desired in accordance with the command input from theexternal host device 80.

FIG. 9 is a graph showing the inductance characteristics of the rotarytransformer 60. The vertical axis represents in logarithmic form theinductance (mH) (the composition of self-inductance and mutualinductance) of the innermost transformer circuit C1, middle transformercircuit C2, and outermost transformer circuit C3 of the rotarytransformer 60. The horizontal axis represents in logarithmic form thegap length (μm) between the stator-side core 61 and rotor-side core 62of the rotary transformer 60. Here, the number of coil turns in each ofthe transformer circuits C1 to C3 is chosen to be 100.

As can be seen from the graph, generally the inductance of eachtransformer circuit increases with decreasing gap length, and decreaseswith increasing gap length. As a result, the transfer efficiency of thetransformer improves as the gap length is reduced.

Further, when the transformer circuits are compared, it is shown thatthe inductance of the innermost transformer circuit C1 dropssignificantly, compared with the middle and outermost transformercircuits C2 and C3. Accordingly, by using, out of these threetransformer circuits, the middle and outermost transformer circuits C2and C3 as a two-phase drive power transformer circuit, the unbalancebetween each phase of the drive power can be suppressed. Here, when theinnermost transformer circuit C1 is used for transmitting the detectionsignal generated at the monitor electrode 31 c, the “unbalance” problemdoes not occur.

The above description has dealt with an example of a double drive typeultrasonic motor in which both the stator-side elastic member 30 and therotor-side elastic member 40 are provided with piezoelectric elements 31and 41, but the invention is also applicable to a single drive typeultrasonic motor in which only the rotor-side elastic member 40 isprovided with a piezoelectric element 41 but the stator-side elasticmember 30 is not provided with a piezoelectric element 31.

Further, the above description has dealt with an example in whichtraveling waves are generated by driving the piezoelectric elements 31and 41 with a two-phase drive signal consisting of a cosine wave and asine wave, but traveling waves can also be generated by driving with adrive signal of three or more phases which matches the arrangement ofthe drive electrodes.

FIG. 10 is a cross sectional side elevation view showing an ultrasonicmotor according to a second embodiment. The ultrasonic motor shown herecomprises an ultrasonic motor main unit 101 made up of mechanical drivemechanisms and an electrical control unit 102 for driving the ultrasonicmotor main unit 1.

The ultrasonic motor main unit 101 comprises a mounting base 103 whichis fixed to the fixed side of a device such as a camera in which theultrasonic motor is disposed, and a cover 120 which is fixed, forexample, by a screw or the like, to the upper part of the mounting base103 so as to cover the upper part of the mounting base 103, andcontains, within an interior space X enclosed by the mounting base 103and cover 120, a stator 105 having a circular periphery and fixed so asto be overlaid on the mounting base 103, a rotating shaft 104 passingthrough the center of the mounting base 103 and stator 105 and supportedrotatably on bearings 107 a and 107 b fixed to the mounting base 103, arotor 106 having a circular periphery and rotatable with the rotatingshaft 104 while being pressed against the stator 105 by means of aBelleville spring 122 fixed to the upper end of the rotating shaft 104,and a rotor power feed unit 121 disposed within a space Y formed betweenthe opposing stator 105 and rotor 106.

Traveling waves are excited in the circumference direction of opposingcontact faces of the stator 105 and rotor 106 and, with the travelingwaves engaging with each other, the phase of one traveling wave isshifted in the positive or negative direction with respect to the phaseof the other traveling wave, thereby causing the engagement position toshift in the forward or backward direction, and by repeating this shiftoperation, the rotor 106 is caused to rotate in the forward or backwarddirection (this operation will be described in detail later) Acylindrically shaped bearing holder 123 of a substantially hollowstructure is disposed passing through the center of the mounting base103. An upper annular flange portion 123 a of the bearing holder 123 isplaced on the mounting base 103 and fixed by a screw or the like. Thebearings 107 a and 107 b are fitted fixedly in the recesses formed inthe bearing holder 123 on top of which the stator 105 is fixed, forexample, by a screw or the like.

The stator 105 comprises an annular vibrator 105 v formed of apiezoelectric ceramic element, and an annular elastic member 105 eformed of a metal, the vibrator 105 v being bonded to an outercircumferential portion of the back surface (underside in the figure) ofthe elastic member 105 e. The elastic member 105 e comprises an annularcircumferential portion 105 o to the back surface of which is bonded thevibrator 105 v, an annular recessed portion 105 s formed on the surface(upside in the figure) inward of the outer circumferential portion 105o, an annular inner circumferential portion 105 i protruding in theopposite direction (downward in the figure) from the outercircumferential portion 105 o and fixed to the bearing holder 123, andan annular middle portion 105 m connecting the upper portion of theinner circumferential portion 105 i with the lower portion of the outercircumferential portion 105 o. The middle portion 105 m is made thinnerthan the outer circumferential portion 105 o and inner circumferentialportion 105 i, and the middle portion 105 m acts to allow the outercircumferential portion 105 o to vibrate easily, and serves to suppressthe transmission of vibrations from the outer circumferential portion105 o to the inner circumferential portion 105 i.

The rotor 106 comprises a rotor-side vibrator 106 v formed of the samematerial as the stator-side vibrator 105 v and having the same shape asthe latter, and a rotor-side elastic member 106 e formed of the samematerial as the stator-side elastic member 105 e and having the sameshape as the latter, the rotor-side vibrator 106 v being bonded to anouter circumferential portion of the the back surface (upside in thefigure) of the elastic member 106 e.

In this way, in the present embodiment, since the two elastic members105 e and 106 e are identical in shape, the stator 105 and rotor 106have the same natural frequency of vibration. Furthermore, the twoelastic members 105 e and 106 e are formed of the same material tofurther ensure that the stator 105 and rotor 106 have the same naturalfrequency of vibration, and in addition, since the two vibrators 105 vand 106 v are formed of the same material and are identical in shape,the natural frequency of vibration of the stator 105 is identical withthat of the rotor 106.

The rotor 106 is disposed so that the rotor-side recessed portion 106 sfaces the stator-side recessed portion 105 s. Further, as with thestator 105, the vibrator 106 v is bonded to an outer circumferentialportion 106 o of the back surface (upside in the figure) of the rotor106, and the thin middle portion 106 m connecting between the outercircumferential portion 106 o and inner circumferential portion 106 i ofthe rotor 106, like the thin middle portion 105 m of the stator 105,acts to allow the outer circumferential portion 106 o to vibrate easilyand serves to suppress the transmission of vibrations from the outercircumferential portion 106 o to the inner circumferential portion 106i.

The rotating shaft 104 passes through the center of the bearing holder123, stator 105, and rotor 106, and is supported rotatably on thebearings 107 a and 107 b. An annular first flange portion 124 is formedupwardly of the position where the rotating shaft 104 is supported onthe upper bearing 107 a, and an annular gap ring 130 is inserted betweenthe first flange portion 124 and the inner ring of the upper bearing 107a.

The lower part of the rotating shaft 104 protrudes downward from thebearing holder 123 and outward from the mounting base 103. Thisprotruding lower part is fitted with a snap ring 127, for example, aC-ring, in a vertically movable manner, and a bush 128 is insertedbetween the snap ring 127 and the inner ring of the lower bearing 107 b.

This snap ring 127 is fitted onto the rotating shaft 104 after therotating shaft 104 in the rotor-side assembly (which includes therotating shaft 104, the rotor 106, and the rotor-side core 141 of therotary transformer 121 described later) is inserted from the upwarddirection in the figure into the stator-side assembly (which includesthe bearings 107 a, 107 b, the bearing holder 123, the stator 105, andthe stator-side core 140 of the rotary transformer 121 described later).

The upward movement of the rotating shaft 104 is limited by the snapring 127 and the bush 128, while the downward movement is limited by thegap ring 130. That is, the snap ring 127, the bush 128, and the gap ring130 together limit the vertical movements of the rotating shaft 104.Further, the vertical positioning of the rotating shaft 104 is definedby the thickness of the gap ring 130.

An annular second flange portion 129 is formed in the upper part therotating shaft 104, and an umbrella-shaped Belleville spring 122 isscrewed to the upper part of the second flange portion 129. A pluralityof loosely engaging grooves 129 a extending in the radial direction areformed at equally spaced intervals around an outer circumferential faceof the second flange 129.

Between the Belleville spring 122 and the rotor-side elastic member 106e is disposed an annular spring base 125 through which the second flangeportion 129 is disposed in slidable fashion (clearance fit to the secondflange portion 129). An upper outer circumferential portion of thespring base 125 is made to butt against an outer circumferential edge ofthe Belleville spring 122. An annular plate 126 is fixed to an upperinner circumferential portion of the spring base 125, and a plurality ofequally spaced protrusions 126 a, which loosely engage with the looselyengaging grooves 129 a, are formed around the inner circumference of theplate 126. Thus, the spring base 125 is made rotatable with the rotatingshaft 104 and movable vertically in sliding fashion.

An annular recessed portion 125 a is formed in a lower innercircumferential portion of the spring base 125, with which annularrecessed portion 125 a the inner circumferential portion 106 i of therotor-side elastic member 106 e engages to position in the axialdirection (centering with respect to the axial center of the rotatingshaft 104), and in this condition, the inner circumferential portion 106i is fixed to the spring base 125 by means of a screw or the like. Thus,the rotor 106 is capable of rotating with the spring base 125 androtating shaft 104, and is, at all times, biased downward by theBelleville spring 122 via the spring base 125.

The stator-side elastic member 105 e includes an annular protrusion 105p with a narrow width, formed integrally with the surface of the outercircumferential portion 105 o, and likewise, the rotor-side elasticmember 106 e includes an annular protrusion 106 p with a narrow width,formed integrally with the surface of the outer circumferential portion106 o. These protrusions 105 p and 106 p are identical in shape andarranged opposite each other. When the stator- and rotor-side elasticmembers 105 e and 106 e were pressed together with their entire surfacescontacting each other, most of the vibrations at the exciting side wouldbe transmitted to the other side and the amount of displacement of thetraveling wave would decrease, but here, the protrusions 105 p and 106 pformed on the stator- and rotor-side elastic members 105 e and 106 eserve to prevent the contact faces from spreading over the entiresurfaces, as a result of which the proper traveling wave can bemaintained on the stator- and rotor-side protrusions 105 p and 106 p.

An annular buffering friction member 112 formed, for example, from aresin or the like is interposed between the stator-side protrusion 105 pand the rotor-side protrusion 106 p, and these protrusions are heldpressed on the buffering friction member 112. That is, the rotor-sideprotrusion 106 p is pressed against the stator-side protrusion 105 p viathe buffering friction member 112 by the elastic force of the Bellevillespring 122.

The buffering friction member 112 here performs the function of avibration low pass filter, and acts to make it difficult for thetraveling wave generated on the stator-side protrusion 105 p or therotor-side protrusion 106 p to be transmitted to the other side, thuspreventing interference between the traveling wave on the stator-sideelastic member 105 e and the traveling wave on the rotor-side elasticmember 106 e and ensuring generation of the proper traveling wave oneach side, while at the same time, preventing the metal portions (theprotrusions 105 p and 106 p) from directly contacting each other,thereby avoiding the generation of abnormal noise and improving thedurability of the press-contact faces. Here, the buffering frictionmember 112 may be fixed to one or the other of the protrusions, or maybe just interposed between the protrusions 105 p and 106 p without beingfixed to either of them.

It should particularly be noted in the present embodiment that the spaceY formed between the opposing stator- and rotor-side recessed portions105 s and 106 s is utilized to accommodate the non-contact rotary powerfeed unit 121 used as the rotor power feed unit. In the presentembodiment, a rotary transformer is used for the non-contact power feedunit 121.

The rotary transformer 121 comprises an annular stator-side core 140 anda rotor-side core 141 identical in shape to the stator-side core 140 anddisposed opposite it. The stator-side core 140 is bonded to the uppersurface of an annular core plate 131 press-fitted to the innercircumferential face of the inner circumferential portion 105 i of thestator-side elastic member 105 e, while the rotor-side core 141 isbonded to the back surface of an annular third flange portion 132 formedbetween the first and second flange portions 124 and 129 of the rotatingshaft 104. The gap between the stator-side core 140 and the rotor-sidecore 141 is set at a predetermined value defined by the thickness of thegap ring 127.

Four annular grooves are formed in concentric fashion on each of thestator- and rotor-side cores 141 and 142, with the annular grooves onone side facing the corresponding annular grooves on the other side, andstator-side coils 141 a to 141 d and rotor-side coils 142 a to 142 d aredisposed in the respective grooves. The stator-side coils 141 a and 141b are primary coils, the former for supplying a first component of asecond drive signal and the latter for supplying a second component ofthe second drive signal, and the stator-side coils 141 c and 141 d are aprimary coil for grounding and a spare primary coil used when the motoris a three-phase ultrasonic motor. The rotor-side coils 142 a to 142 dfacing these stator-side coils 141 a to 141 d are a secondary coil forsupplying the first component of the second drive signal, a secondarycoil for supplying the second component of the second drive signal, asecondary coil for grounding, and a spare secondary coil, respectively.Preferably, the first and second components are sine waves, 90 degreesapart in phase, but they may be supplied as rectangular waves.

FIG. 11 is a plan view showing one side of the stator-side vibrator 105v or the rotor-side vibrator 106 v. The vibrator 105 v or 106 vcomprises an annular piezoelectric ceramic plate CM, and four firstelectrodes S1 to S4 and four second electrodes C1 to C4 formed on oneside of the piezoelectric ceramic plate CM. The first electrodes S1 toS4 and C1 to C4 are equally spaced apart by 36 degrees in terms ofmechanical angle so that the vibrator generates a standing wave of fivewavelengths (5λ) around the entire circumference. The first electrodesS1 to S4 and the second electrodes C1 to C4 are polarized in advance sothat the polarization direction across the thickness reverses betweenadjacent regions (as marked with + and − in the figure). The electrodesS1 to S4 and C1 to C4 are formed without leaving any gap to the innercircumferential edge or outer circumferential edge of the annularpiezoelectric ceramic plate CM.

FIG. 12 is a plan view showing the other side of the stator-sidevibrator 105 v or the rotor-side vibrator 106 v. On the other side ofthe piezoelectric ceramic plate CM are formed a first-side electrode SSopposite the entire region where the electrodes S1 to S4 are formed onthe one side of the piezoelectric ceramic plate CM, and a second-sideelectrode CC opposite the entire region where the second electrodes C1to C4 are formed on the one side of the piezoelectric ceramic plate CM.Between the first-side electrode SS and the second-side electrode CC areformed feedback electrodes FB and FB′ opposing each other and eachhaving an angle of 18 degrees in terms of mechanical angle or 90 degreesin terms of electrical angle.

Referring back to FIG. 10, the entire surface of the one side of thestator-side vibrator 105 v is bonded to the back surface of the metalelastic member 105 e by means of an adhesive or a conductive adhesive.Generally, even when a non-conductive adhesive is used, electricalconduction occurs between the two members with microscopic roughenedportions on the machined surfaces contacting each other because theadhesive thickness is thin. The metal elastic member 105 e is connectedelectrically to the mounting base 103, and as a result, the electrodesS1 to S4, FB, and C1 to C4 formed on the one side of the stator-sidevibrator 105 v are connected to ground.

On the other hand, the first-side electrode SS and second-side electrodeCC of the stator-side vibrator 105 v are connected to a drive circuit116.

The drive circuit 116 applies, between the ground and the first-sideelectrode SS of the stator-side vibrator 105 v, a sinusoidal voltagesignal as a first component of a first drive signal and, between theground and the second-side electrode CC, a sinusoidal voltage signal asa second component of the first drive signal which has a 90-degree phasedifference with respect to the first component. The drive circuit 16 issupplied with a piezoelectric voltage signal representing thedisplacement detection signal produced between the feedback electrodesFB and FB′ in response to the amount of displacement of the vibrator 5 vand, based on the supplied displacement detection signal, the phase andfrequency of the sinusoidal voltage signals to be supplied to thestator-side vibrator 105 v are maintained constant.

On the other hand, the entire surface of the one side of the rotor-sidevibrator 106 v is bonded to the back surface of the metal elastic member106 e by means of an adhesive or a conductive adhesive. The metalelastic member 106 e is electrically connected to the mounting base 103via internal wiring 114 c, rotor-side coil 142 c, stator-side coil 141c, and internal wiring 114 cc electrically connected in sequence to themetal elastic member 106 e, and as a result, the electrodes S1 to S4,FB, and C1 to C4 formed on the one side of the rotor-side vibrator 106 vare connected to ground.

On the other hand, the first-side electrode SS of the rotor-sidevibrator 106 v is connected to the drive circuit 116 via internal wiring114 a, rotor-side coil 142 a, and stator-side coil 141 a electricallyconnected in sequence to the first-side electrode SS. The second-sideelectrode CC of the rotor-side vibrator 106 v is connected to the drivecircuit 116 via internal wiring 114 b, rotor-side coil 142 b, andstator-side coil 141 b electrically connected in sequence to thesecond-side electrode CC.

The drive circuit 116 applies, between the ground and the first-sideelectrode SS of the rotor-side vibrator 106 v, a sinusoidal voltagesignal as the first component of the second drive signal and, betweenthe ground and the second-side electrode CC, a sinusoidal voltage signalas the second component of the second drive signal which has a 90-degreephase difference with respect to the first component.

Next, the operation of the thus constructed ultrasonic motor will bedescribed in further detail along with the operation of the drivecircuit 116. When the first drive power as the first component of thefirst drive signal is supplied from the drive circuit 116 to thefirst-side electrode SS of the vibrator 5 v, the vibrator 105 v isexcited and a standing wave of five wavelengths is generated on thesurface of the elastic member along the circumference direction thereof.

Here, since the outer circumferential portion 105 o to which thevibrator 105 v is bonded is allowed to vibrate by the presence of thethin middle portion 105 m, as earlier described, the standing wave isgenerated on the surface of the outer circumferential portion 105 o ofthe stator 105.

When the second drive power as the second component of the first drivesignal, which has a 90-degree phase difference with respect to the firstdrive power, is supplied to the second-side electrode CC, a standingwave of five wavelengths is generated on the surface of the outercircumferential portion 105 o of the stator 105 along the circumferencedirection thereof. Since the first-side electrode SS and the second-sideelectrode CC are spaced apart from each other by 90 degrees in terms ofelectrical angle, as earlier noted, the two standing waves are 90degrees apart in phase.

Accordingly, when the first drive power and second drive power from thedrive circuit 116 are simultaneously supplied to the stator-sidevibrator 105 v, a traveling rotational wave due to the sum wave isgenerated on the surface of the outer circumferential portion 105 o ofthe stator 105. Likewise, when the first drive power and second drivepower are simultaneously supplied to the rotor-side vibrator 106 v viathe rotary transformer 121, a traveling rotational wave due to the sumwave is generated on the surface of the outer circumferential portion106 o of the rotor 106.

In the present embodiment, the control circuit 117 first controls thedrive circuit 116 so that the drive circuit 116 supplies the two-phasefirst drive signal of frequency f to the stator-side vibrator 105 v andthe two-phase second drive signal of the same frequency f to therotor-side vibrator 106 v.

Since the stator 105 and rotor 106 have the same natural frequency ofvibration, as earlier noted, when the frequency f is set equal or nearlyequal to the natural frequency of vibration of the stator 105 and rotor106, the traveling wave A generated on the protrusion 105 p of thestator-side outer circumferential portion 105 o becomes identical withthe traveling wave B generated on the protrusion 106 p of the rotor-sideouter circumferential portion 106 o, and the two waves thus lock witheach other, as shown in FIG. 13, just as gears engage with each other.As a result, the rotor 106 is locked in the stopped condition. In thepresent embodiment, the frequency f is set, for example, to 50 kHz.

In this condition, when the control circuit 117 controls the drivecircuit 116 so that the drive circuit 116 supplies the two-phase firstdrive signal of frequency f to the stator-side vibrator 105 v and thetwo-phase second drive signal of frequency (f+Δf) to the rotor-sidevibrator 106 v, then the rotor-side traveling wave B tries to advancerelative to the stator-side traveling wave A with the two waves engagingwith each other, as a result of which the engagement lock position ofthe traveling waves A and B moves in the propagating direction of thetraveling waves, causing the rotor 106 to rotate in the forwarddirection while the stator 105 is held stationary.

On the other hand, when the control circuit 117 controls the drivecircuit 116 so that the drive circuit 116 supplies the two-phase firstdrive signal of frequency f to the stator-side vibrator 105 v and thetwo-phase second drive signal of frequency (f−Δf) to the rotor-sidevibrator 106 v, then the rotor-side traveling wave B tries to lagrelative to the stator-side traveling wave A with the two waves engagingwith each other, as a result of which the engagement lock position ofthe traveling waves A and B moves in the direction opposite to thepropagating direction of the traveling waves, causing the rotor 106 torotate in the backward direction while the stator 105 is heldstationary.

Furthermore, when the amount of the frequency change, ∓Δf, is increasedor decreased, the rotational speed of the rotor 106 increases ordecreases. The rotational speed (rpm) defined by the value of ±Δfindicates the synchronous rotational speed determined by60×Δf/wavelength, for example, in the five-wave type of FIG. 11, 60×⅕=12rpm when Δf is 1 Hz.

As described above, in the present embodiment, the two elastic members105 e and 106 e are made identical in shape so that the stator 105 androtor 106 will have the same natural frequency of vibration, and as aresult, when signals whose frequencies are equal to or nearly equal tothe resonant frequency of the stator 105 and rotor 106 are supplied tothe respective vibrators 105 v and 106 v, the generated traveling wavesA and B lock with each other, just as gears engage with each other, thuslocking the rotor 106 in the stopped condition, and when the phase ofone signal is shifted in the positive or negative direction with respectto the phase of the other signal, the engagement lock position shifts inthe forward or backward direction and, by repeating this shiftoperation, the rotor 106 is caused to rotate in the forward or backwarddirection.

In this way, in the ultrasonic motor whose stator 105 and rotor 106 areprovided with the respective vibrators 105 v and 106 v, when the twoelastic members 105 e and 106 e are made identical in shape, therotation of the rotor 106 can be regulated in a well controlled manner.

It must be noted, however, that the natural frequencies of the stator105 and rotor 106 shift because of changes in temperature, humidity,load, applied pressure, drive power, etc., and when there is adifference in amount of shift in the natural frequency between thestator 105 and rotor 106, the locked condition of the traveling waves Aand B could not be maintained, but since the two vibrators 105 v and 106v are formed into the same shape and of the same material, and since thetwo elastic members 105 e and 106 e are also formed into the same shapeand of the same material, their natural frequencies of vibration areshifted in the same direction by the same amount.

Thus, according to the present embodiment, the rotor 106 can beregulated at all times in a well controlled manner by maintaining thetraveling waves A and B in the properly locked condition.

Furthermore, since the same components can be used between the vibrators105 v and 106 v and between the elastic members 105 e and 106 e, theircosts can be reduced.

Moreover, according to the present embodiment, since the space Y formedbetween the opposing recessed portions 105 s and 106 s of the elasticmembers 105 e and 106 e of the same shape is utilized to accommodate therotary transformer 121 used as the power feed unit for supplying asignal of prescribed frequency to the rotor-side vibrator 106 v, notonly can the size of the motor be reduced in the axial directioncompared with the prior art, but also the need for the matching betweenthe cover 120 and the motor mechanism, which is difficult to achieve, iseliminated, contributing to the reduction of the manufacturing costs.

FIG. 14 is a cross sectional side elevation view showing an ultrasonicmotor according to a third embodiment. The ultrasonic motor of the thirdembodiment differs from that of the second embodiment in that theBelleville spring 122 is replaced by a compression spring 144 as thebiasing means for pressing the stator and rotor together, and in thatthe rotating shaft 104 is supported on a single bearing 107 a. Withthese changes, various modifications are made.

That is, the bearing 107 a is fixedly fitted in the mounting base 103and the inner circumferential portion 105 i of the stator-side elasticmember 105 e, and the rotating shaft 104 is rotatably supported on thisbearing 107 a. The rotating shaft 104 has only one flange portion 143 inan upper part thereof, and the inner circumferential portion 106 i ofthe rotor-side elastic member 106 e is press-fitted onto this flangeportion 143.

The lower part of the rotating shaft 104 is fitted with a snap ring 127such as a C-ring, and the compression spring 144 is disposed between thesnap ring 127 and the bearing 107 a via a bush 128. The rotating shaft104 and the rotor 106 are at all times biased downward by thecompression spring 144, and the stator-side protrusion 105 p and therotor-side protrusion 106 p are pressed against each other via thebuffering friction material 112, as in the second embodiment.

As in the second embodiment, the stator-side core 140 is bonded to theupper surface of the inner circumferential portion 105 i of thestator-side elastic member 105 e, and the rotor-side core 141 to theback surface of the inner circumferential portion 106 i of therotor-side elastic member 106 e, and the rotary transformer 121 isdisposed utilizing the space Y.

It will be appreciated that the same effect as obtained in the secondembodiment can be achieved with the above construction.

FIG. 15 is a cross sectional side elevation view showing an ultrasonicmotor according to a fourth embodiment. The ultrasonic motor of thefourth embodiment differs from that of the third embodiment in that thenon-contact rotary power feed unit 121 is replaced by a contact rotarypower feed unit, i.e., a brush contact power feed unit 151, as the rotorpower feed unit disposed in the space Y.

The brush contact power feed unit 151 comprises a stator-side power feedunit 151A and a rotor-side power feed unit 151B. The rotor-side powerfeed unit 151B comprises an annular base 154 fixed to the back surfaceof the inner circumferential portion 106 i of the rotor-side elasticmember 106 e so as to be coaxial with the rotating shaft 104, and fourannular conductors 155 a to 155 d formed in concentric fashion on theback surface of the base 154.

On the other hand, the stator-side power feed unit 151A comprises abrush base 152 fixed to the upper surface of the inner circumferentialportion 105 i of the stator-side elastic member 105 e so as to face aportion of the annular base 154, and four positive conductive brushes153 a to 153 d attached to the upper surface of the brush base 152 andcontacting the respective conductors 155 a to 155 d in rubbing fashion.The positive conductive brushes 153 a to 153 d are, respectively, abrush for supplying the first component of the second drive signal, abrush for supplying the second component of the second drive signal, agrounding brush, and a spare brush.

The first-side electrode SS of the rotor-side vibrator 106 v isconnected to the drive circuit 116 via the internal wiring 114 a,conductor 155 a, and conductive brush 153 a electrically connected insequence to the first-side electrode SS. The second-side electrode CC ofthe rotor-side vibrator 106 v is connected to the drive circuit 116 viathe internal wiring 114 b, conductor 155 b, and conductive brush 153 belectrically connected in sequence to the second-side electrode CC. Theelectrodes S1 to S4, FB, and C1 to C4 formed on one side (underside inthe figure) of the rotor-side vibrator 106 v are electrically connectedto the mounting base 103, and hence to ground, via the metal elasticmember 106 e, internal wiring 114 c, conductor 155 c, conductive brush153 c, and internal wiring 114 cc electrically connected in sequence tothese electrodes.

It will be appreciated that the same effect as obtained in the thirdembodiment can be achieved with the above construction.

The present invention has been described above in connection with thepreferred embodiments thereof, but the invention is not limited to theparticular embodiments described above, and it will be recognized thatvarious modifications can be made without departing from the scope ofthe invention, for example, in the fourth embodiment, the conductivebrushes 153 a to 153 d are fixed to the stator side and the conductors155 a to 155 d to the rotor side, but conversely, the conductive brushes153 a to 153 d may be fixed to the rotor side and the conductors 155 ato 155 d to the stator side.

Further, the above embodiments have been described as applied toultrasonic motors which generate traveling waves by applying two-phasedrive signals, but these embodiment are equally applicable to ultrasonicmotors which generate traveling waves by applying three-phase drivesignals. In fact, in the above embodiments, the spare coils 141 d and142 d or the spare brush 153 d and spare conductor 155 d are included,as described above, to provide for three-phase drive signals.

ADVANTAGEOUS EFFECTS OF THE INVENTION

As described in detail above, according to the present invention, sincepower is supplied to the rotor-side vibrating element using a rotarytransformer and thus achieving a space saving, stable, and non-contactpower feed construction, a compact and high-output ultrasonic motor canbe realized.

When driving the rotor-side vibrating element with N-phase drive power,since, of the plurality of concentrically arranged transformer circuits,N transformer circuits starting from the outermost transformer circuitare used to supply power, the unbalance between each phase of the drivepower can be suppressed.

Further, by using a rotary transformer whose step-up ratio Nr is largerthan 1, the rotary transformer can be used as a substitution for a fixedtransformer for the rotor-side vibrating element, and thus the number ofcomponents can be reduced.

Moreover, by setting the ratio Nr/Ns between the step-up ratio Nr of therotary transformer and the step-up ratio Ns of the fixed transformer forthe stator-side vibrating element within the range of 0.5 to 2, theunbalance of the supply power or drive voltages to the stator-side androtor-side vibrating elements can be eliminated.

Furthermore, according to the ultrasonic motor of the invention, sincethe two elastic members are made substantially identical in shape, thestator and rotor can be constructed to have the same natural frequencyof vibration. Accordingly, by supplying signals whose frequencies areequal to or nearly equal to the resonant frequency of the stator androtor to the respective vibrators, the traveling waves generated ontheir press-contact faces can be made to lock with each other. As aresult, by successively shifting the engagement lock position of thetraveling waves, the rotation of the rotor can be regulated in a wellcontrolled manner.

Further, since the space formed between the opposing recessed portionsof the elastic members is utilized to accommodate the power feed unitfor supplying a signal of prescribed frequency to the rotor-sidevibrator thus eliminating the need for particularly providing a spacefor accommodating the power feed unit along the axial direction of themotor, not only can the size of the motor be reduced in the axialdirection compared with the prior art, but also the need for thematching between the cover and the motor mechanism, which is difficultto achieve, is eliminated, contributing to reducing the manufacturingcosts. Furthermore, since the same components can be used between therespective elastic members, their costs can be reduced.

What is claimed is:
 1. An ultrasonic motor comprising: a stator-sideelastic member having a rotationally symmetrical press-contact face; arotor-side elastic member having a rotationally symmetricalpress-contact face facing the press-contact face of the stator-sideelastic member, the rotor-side elastic member being supported so as tobe angularly displaceable about a rotation symmetry axis; a stator-sidevibrating element for generating a rotational displacement wave offrequency Fs on the press-contact face of the stator-side elasticmember; a rotor-side vibrating element for generating a rotationaldisplacement wave of frequency Fr on the press-contact face of therotor-side elastic member, the rotor-side elastic member being angularlydisplaced at a rotational speed proportional to a frequency differenceΔF between the frequency Fs and the frequency Fr; and a rotarytransformer for supplying drive power having a phase differenceequivalent to N phases (N is an integer equal to or more than 2) to therotor-side vibrating element which is angularly displaced together withthe rotor-side elastic member, the rotary transformer including morethan N transformer circuits concentrically arranged, N transformercircuits which are sequentially arranged inwardly from an outermost onebeing used as drive power transformer circuits.
 2. The ultrasonic motorof claim 1, wherein an innermost one of the transformer circuits is usedas a detection signal transformer circuit for transmitting a detectionsignal resulting from the detection of the rotational displacement wavegenerated on the press-contact face of the rotor-side elastic member. 3.The ultrasonic motor of claim 2, wherein the stator-side vibratingelement and the rotor-side vibrating element, respectively, arerotationally symmetrical piezoelectric elements attached to surfaces ofthe stator-side and rotor-side elastic members, which surfaces areopposite to the press-contact faces thereof, first and second driveelectrodes for two-phase driving and a monitor electrode for detecting avibrating wave are formed on a surface of each of the piezoelectricelements, the first and second drive electrodes are respectivelyconnected to the drive power transformer circuits, and the monitorelectrode is connected to the detection signal transformer circuit. 4.The ultrasonic motor of claim 3, wherein a non-magnetic material forsuppressing magnetic coupling is interposed between a detection coreforming the detection signal transformer circuit and a drive coreforming the drive power transformer circuits.
 5. An ultrasonic motorcomprising: a stator-side elastic member having a rotationallysymmetrical press-contact face; a rotor-side elastic member having arotationally symmetrical press-contact face facing the press-contactface of the stator-side elastic member, the rotor-side elastic memberbeing supported so as to be angularly displaceable about a rotationsymmetry axis; a rotor-side vibrating element for generating arotational displacement wave on the press-contact face of the rotor-sideelastic member, the rotor-side elastic member being angularly displacedby the rotational displacement wave; and a rotary transformer forsupplying drive power having a phase difference equivalent to N phases(N is an integer equal to or more than 2) to the rotor-side vibratingelement which is angularly displaced together with the rotor-sideelastic member, the rotary transformer including more than N transformercircuits concentrically arranged, N transformer circuits which aresequentially arranged inwardly from an outermost one being used as drivepower transformer circuits.
 6. An ultrasonic motor comprising: astator-side elastic member having a rotationally symmetricalpress-contact face; a rotor-side elastic member having a rotationallysymmetrical press-contact face facing the press-contact face of thestator-side elastic member, the rotor-side elastic member beingsupported so as to be angularly displaceable about a rotation symmetryaxis; a stator-side vibrating element for generating a rotationaldisplacement wave of frequency Fs on the press-contact face of thestator-side elastic member; a rotor-side vibrating element forgenerating a rotational displacement wave of frequency Fr on thepress-contact face of the rotor-side elastic member, the rotor-sideelastic member being angularly displaced at a rotational speedproportional to a frequency difference ΔF between the frequency Fs andthe frequency Fr; and a rotary transformer, disposed on the rotationsymmetry axis side with respect to the press-contact faces of thestator- and rotor-side elastic members, for supplying power to therotor-side vibrating element which is angularly displaced together withthe rotor-side elastic member, a step-up ratio Nr of the rotarytransformer being larger than
 1. 7. The ultrasonic motor of claim 6,further comprising: a fixed transformer for supplying power to thestator-side vibrating element, wherein a ratio Nr/Ns between the step-upratio Nr of the rotary transformer and the step-up ratio Ns of the fixedtransformer satisfies a relation of 0.5≦Nr/Ns≦2.
 8. An ultrasonic motorcomprising: a stator-side elastic member having a rotationallysymmetrical press-contact face; a rotor-side elastic member having arotationally symmetrical press-contact face facing the press-contactface of the stator-side elastic member, the rotor-side elastic memberbeing supported so as to be angularly displaceable about a rotationsymmetry axis; a rotor-side vibrating element for generating arotational displacement wave on the press-contact face of the rotor-sideelastic member, the rotor-side elastic member being angularly displacedby the rotational displacement wave; and a rotary transformer forsupplying power to the rotor-side vibrating element which is angularlydisplaced together with the rotor-side elastic member, a step-up ratioNr of the rotary transformer being larger than
 1. 9. An ultrasonic motorcomprising: a stator having a stator-side elastic member and astator-side vibrator attached to the stator-side elastic member; and arotor having a rotor-side elastic member facing and pressed against thestator-side elastic member and a rotor-side vibrator attached to therotor-side elastic member, the vibrators being caused to vibrate bysupplying signals of prescribed frequency to the respective vibrators,the rotor being driven by means of traveling waves generated on thepress-contact faces of the respective elastic members by the vibrations,wherein the stator-side elastic member and the rotor-side elasticmembers are respectively provided with recessed portions opposing eachother and are formed into substantially the same shape, and a power feedunit for supplying the signal of the prescribed frequency to therotor-side vibrator is disposed within a space formed between theopposing recessed portions.
 10. The ultrasonic motor of claim 9, whereinthe power feed unit is a rotary transformer.
 11. The ultrasonic motor ofclaim 9, wherein the power feed unit is a brush contact power feed unitcomprising a conductive brush attached to either the stator or rotorside and a conductor provided at the other side, for contacting theconductive brushes in rubbing fashion.